Python yield 用法

yield 简单说来就是一个生成器，生成器是这样一个函数，它记住上一次返回时在函数体中的位置。对生成器函数的第二次（或第 n 次）调用跳转至该函数中间，而上次调用的所有局部变量都保持不变。

• 生成器 是 一个函数

函数的所有参数都会保留

• 第二次调用 此函数 时

使用的参数是前一次保留下的.

• 生成器还“记住”了它在流控制构造

生成器不仅“记住”了它数据状态。 生成器还“记住”了它在流控制构造（在命令式编程中，这种构造不只是数据值）中的位置。由于连续性使您在执行框架间任意跳转，而不总是返回到直接调用者的上下文（如同生成器那样），因此它仍是比较一般的。

当你问生成器要一个数时，生成器会执行，直至出现yield 语句，生成器把yield 的参数给你，之后生成器就不会往下继续运行。 当你问他要下一个数时，他会从上次的状态开始运行，直至出现yield语句，把参数给你，之后停下。如此反复直至退出函数。

<span class="co1" style="padding: 0px; margin: 0px; color: rgb(128, 128, 128); font-style: italic;">#生成全排列</span>
<span class="kw1" style="padding: 0px; margin: 0px; color: rgb(177, 177, 0);">def</span> perm<span class="br0" style="padding: 0px; margin: 0px; color: rgb(102, 204, 102);">(</span>items, n=<span class="kw2" style="padding: 0px; margin: 0px; color: rgb(0, 0, 0); font-weight: bold;">None</span><span class="br0" style="padding: 0px; margin: 0px; color: rgb(102, 204, 102);">)</span>:
    <span class="kw1" style="padding: 0px; margin: 0px; color: rgb(177, 177, 0);">if</span> n <span class="kw1" style="padding: 0px; margin: 0px; color: rgb(177, 177, 0);">is</span> <span class="kw2" style="padding: 0px; margin: 0px; color: rgb(0, 0, 0); font-weight: bold;">None</span>:
        n = <span class="kw2" style="padding: 0px; margin: 0px; color: rgb(0, 0, 0); font-weight: bold;">len</span><span class="br0" style="padding: 0px; margin: 0px; color: rgb(102, 204, 102);">(</span>items<span class="br0" style="padding: 0px; margin: 0px; color: rgb(102, 204, 102);">)</span>
    <span class="kw1" style="padding: 0px; margin: 0px; color: rgb(177, 177, 0);">for</span> i <span class="kw1" style="padding: 0px; margin: 0px; color: rgb(177, 177, 0);">in</span> <span class="kw2" style="padding: 0px; margin: 0px; color: rgb(0, 0, 0); font-weight: bold;">range</span><span class="br0" style="padding: 0px; margin: 0px; color: rgb(102, 204, 102);">(</span><span class="kw2" style="padding: 0px; margin: 0px; color: rgb(0, 0, 0); font-weight: bold;">len</span><span class="br0" style="padding: 0px; margin: 0px; color: rgb(102, 204, 102);">(</span>items<span class="br0" style="padding: 0px; margin: 0px; color: rgb(102, 204, 102);">)</span><span class="br0" style="padding: 0px; margin: 0px; color: rgb(102, 204, 102);">)</span>:
        v = items<span class="br0" style="padding: 0px; margin: 0px; color: rgb(102, 204, 102);">[</span>i:i+<span class="nu0" style="padding: 0px; margin: 0px; color: rgb(204, 102, 204);">1</span><span class="br0" style="padding: 0px; margin: 0px; color: rgb(102, 204, 102);">]</span>
        <span class="kw1" style="padding: 0px; margin: 0px; color: rgb(177, 177, 0);">if</span> n == <span class="nu0" style="padding: 0px; margin: 0px; color: rgb(204, 102, 204);">1</span>:
            <span class="kw1" style="padding: 0px; margin: 0px; color: rgb(177, 177, 0);"><span class="search_hit" style="padding: 0px; margin: 0px; color: rgb(51, 51, 51); background-color: rgb(255, 255, 153);">yield</span></span> v
        <span class="kw1" style="padding: 0px; margin: 0px; color: rgb(177, 177, 0);">else</span>:
            rest = items<span class="br0" style="padding: 0px; margin: 0px; color: rgb(102, 204, 102);">[</span>:i<span class="br0" style="padding: 0px; margin: 0px; color: rgb(102, 204, 102);">]</span> + items<span class="br0" style="padding: 0px; margin: 0px; color: rgb(102, 204, 102);">[</span>i+<span class="nu0" style="padding: 0px; margin: 0px; color: rgb(204, 102, 204);">1</span>:<span class="br0" style="padding: 0px; margin: 0px; color: rgb(102, 204, 102);">]</span>
            <span class="kw1" style="padding: 0px; margin: 0px; color: rgb(177, 177, 0);">for</span> p <span class="kw1" style="padding: 0px; margin: 0px; color: rgb(177, 177, 0);">in</span> perm<span class="br0" style="padding: 0px; margin: 0px; color: rgb(102, 204, 102);">(</span>rest, n-<span class="nu0" style="padding: 0px; margin: 0px; color: rgb(204, 102, 204);">1</span><span class="br0" style="padding: 0px; margin: 0px; color: rgb(102, 204, 102);">)</span>:
                <span class="kw1" style="padding: 0px; margin: 0px; color: rgb(177, 177, 0);"><span class="search_hit" style="padding: 0px; margin: 0px; color: rgb(51, 51, 51); background-color: rgb(255, 255, 153);">yield</span></span> v + p
 
<span class="co1" style="padding: 0px; margin: 0px; color: rgb(128, 128, 128); font-style: italic;">#生成组合</span>
<span class="kw1" style="padding: 0px; margin: 0px; color: rgb(177, 177, 0);">def</span> comb<span class="br0" style="padding: 0px; margin: 0px; color: rgb(102, 204, 102);">(</span>items, n=<span class="kw2" style="padding: 0px; margin: 0px; color: rgb(0, 0, 0); font-weight: bold;">None</span><span class="br0" style="padding: 0px; margin: 0px; color: rgb(102, 204, 102);">)</span>:
    <span class="kw1" style="padding: 0px; margin: 0px; color: rgb(177, 177, 0);">if</span> n <span class="kw1" style="padding: 0px; margin: 0px; color: rgb(177, 177, 0);">is</span> <span class="kw2" style="padding: 0px; margin: 0px; color: rgb(0, 0, 0); font-weight: bold;">None</span>:
        n = <span class="kw2" style="padding: 0px; margin: 0px; color: rgb(0, 0, 0); font-weight: bold;">len</span><span class="br0" style="padding: 0px; margin: 0px; color: rgb(102, 204, 102);">(</span>items<span class="br0" style="padding: 0px; margin: 0px; color: rgb(102, 204, 102);">)</span>    
    <span class="kw1" style="padding: 0px; margin: 0px; color: rgb(177, 177, 0);">for</span> i <span class="kw1" style="padding: 0px; margin: 0px; color: rgb(177, 177, 0);">in</span> <span class="kw2" style="padding: 0px; margin: 0px; color: rgb(0, 0, 0); font-weight: bold;">range</span><span class="br0" style="padding: 0px; margin: 0px; color: rgb(102, 204, 102);">(</span><span class="kw2" style="padding: 0px; margin: 0px; color: rgb(0, 0, 0); font-weight: bold;">len</span><span class="br0" style="padding: 0px; margin: 0px; color: rgb(102, 204, 102);">(</span>items<span class="br0" style="padding: 0px; margin: 0px; color: rgb(102, 204, 102);">)</span><span class="br0" style="padding: 0px; margin: 0px; color: rgb(102, 204, 102);">)</span>:
        v = items<span class="br0" style="padding: 0px; margin: 0px; color: rgb(102, 204, 102);">[</span>i:i+<span class="nu0" style="padding: 0px; margin: 0px; color: rgb(204, 102, 204);">1</span><span class="br0" style="padding: 0px; margin: 0px; color: rgb(102, 204, 102);">]</span>
        <span class="kw1" style="padding: 0px; margin: 0px; color: rgb(177, 177, 0);">if</span> n == <span class="nu0" style="padding: 0px; margin: 0px; color: rgb(204, 102, 204);">1</span>:
            <span class="kw1" style="padding: 0px; margin: 0px; color: rgb(177, 177, 0);"><span class="search_hit" style="padding: 0px; margin: 0px; color: rgb(51, 51, 51); background-color: rgb(255, 255, 153);">yield</span></span> v
        <span class="kw1" style="padding: 0px; margin: 0px; color: rgb(177, 177, 0);">else</span>:
            rest = items<span class="br0" style="padding: 0px; margin: 0px; color: rgb(102, 204, 102);">[</span>i+<span class="nu0" style="padding: 0px; margin: 0px; color: rgb(204, 102, 204);">1</span>:<span class="br0" style="padding: 0px; margin: 0px; color: rgb(102, 204, 102);">]</span>
            <span class="kw1" style="padding: 0px; margin: 0px; color: rgb(177, 177, 0);">for</span> c <span class="kw1" style="padding: 0px; margin: 0px; color: rgb(177, 177, 0);">in</span> comb<span class="br0" style="padding: 0px; margin: 0px; color: rgb(102, 204, 102);">(</span>rest, n-<span class="nu0" style="padding: 0px; margin: 0px; color: rgb(204, 102, 204);">1</span><span class="br0" style="padding: 0px; margin: 0px; color: rgb(102, 204, 102);">)</span>:
                <span class="kw1" style="padding: 0px; margin: 0px; color: rgb(177, 177, 0);"><span class="search_hit" style="padding: 0px; margin: 0px; color: rgb(51, 51, 51); background-color: rgb(255, 255, 153);">yield</span></span> v + c
 
a = perm<span class="br0" style="padding: 0px; margin: 0px; color: rgb(102, 204, 102);">(</span><span class="st0" style="padding: 0px; margin: 0px; color: rgb(255, 0, 0);">'abc'</span><span class="br0" style="padding: 0px; margin: 0px; color: rgb(102, 204, 102);">)</span>
<span class="kw1" style="padding: 0px; margin: 0px; color: rgb(177, 177, 0);">for</span> b <span class="kw1" style="padding: 0px; margin: 0px; color: rgb(177, 177, 0);">in</span> a:
    <span class="kw1" style="padding: 0px; margin: 0px; color: rgb(177, 177, 0);">print</span> b
    <span class="kw1" style="padding: 0px; margin: 0px; color: rgb(177, 177, 0);">break</span>
<span class="kw1" style="padding: 0px; margin: 0px; color: rgb(177, 177, 0);">print</span> <span class="st0" style="padding: 0px; margin: 0px; color: rgb(255, 0, 0);">'-'</span><span class="sy0" style="padding: 0px; margin: 0px; color: rgb(102, 204, 102);">*</span><span class="nu0" style="padding: 0px; margin: 0px; color: rgb(204, 102, 204);">20</span>
<span class="kw1" style="padding: 0px; margin: 0px; color: rgb(177, 177, 0);">for</span> b <span class="kw1" style="padding: 0px; margin: 0px; color: rgb(177, 177, 0);">in</span> a:
    <span class="kw1" style="padding: 0px; margin: 0px; color: rgb(177, 177, 0);">print</span> b

结果如下：

102 pvopf006 ~/test> ./generator.py
abc
--------------------
acb
bac
bca
cab
cba

可以看到，在第一个循环break后，生成器没有继续执行，而第二个循环接着第一个循环执行